The present invention relates generally to inkjet printing, and more particularly to controlling pen to paper spacing within an inkjet printing apparatus.
An inkjet printing apparatus is a type of non-impact printing device that forms characters, symbols, graphics or other images by controllably spraying drops of ink. The apparatus typically includes a cartridge, often called a “pen,” which houses a printhead. The printhead has very small nozzles through which the ink drops are ejected. To print an image the pen is propelled back and forth across a media sheet, while the ink drops are ejected from the printhead in a controlled pattern.
An inkjet printing apparatus may be employed in a variety of devices, such as printers, plotters, scanners, facsimile machines, copiers, and the like. There are various forms of inkjet printheads, known to those skilled in the art, including, for example, thermal inkjet printheads and piezoelectric printheads. Two earlier thermal inkjet ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, currently assigned to the present assignee, The Hewlett-Packard Company of Palo Alto, Calif. In a thermal inkjet printing system, ink flows along ink channels from a reservoir into an array of vaporization chambers. Associated with each chamber are a heating element and a nozzle. A respective heating element is energized to heat ink contained within the corresponding chamber. The corresponding nozzle forms an ejection outlet for the heated ink. As the pen moves across the page, the heating elements are selectively energized causing ink drops to be expelled in a controlled pattern. The ink drops dry on the page shortly after deposition to form a desired image (e.g., text, chart, graphic or other image).
Pen to paper spacing (‘PPS’) is the average normal distance from an outer surface of the printhead to the paper within the print zone. In an inkjet printing apparatus, the ink typically includes a relatively large amount of water. As the wet ink contacts the paper, the water in the ink saturates the paper fibers, causing the fibers to expand, which in turn causes the paper to buckle. Such buckling action also is referred to as cockling. Cockling of the paper tends to cause the paper to bend in an uncontrolled manner downward away from the printhead and upward toward the printhead. Cockling varies the pen to paper spacing (‘PPS’), which reduces print quality. In the extreme an upwardly buckling page contacts a pen nozzle causing ink to smear on the paper. In a worst case scenario an upwardly buckling page in contact with a nozzle damages the nozzle.